What is wrong with my rigging here?
The blend file already contains the enough instances of mesh object and the armature (RIG-Sky), but the instances are not registered to any scene collection. Therefore delete armature and empty(GRP-Sky) objects and then run the following script to link the instances to a collection.
import bpy

for obj in bpy.data.objects:
    if obj.name not in bpy.context.scene.objects.keys():
        bpy.context.scene.collection.objects.link(obj)



Another way: Use Shift M key in the outliner